The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Cross-linking agents, such as borates, titanates or zirconates, are usually incorporated into the fluid composition to control viscosity.
Typically, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through permeable zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.
Difficulties in oil recovery due to thief zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linking agent into a subterranean formation under conditions where the polymer will be cross-linked to produce a gel, thus reducing permeability of the subterranean formation to the driving fluid (gas, water, etc.). Polysaccharide-or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium, and boron based compounds are used in these enhanced oil recovery applications. Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of zones in subterranean formation, are now being used in hotter and deeper wells under a variety of temperature and pH conditions. In these operations the rate of cross-link is critical to the successful generation of viscosity. Frequently the rates of cross-linking with known cross-linking compositions are unacceptable, and new, highly specific compositions are required. Particularly, the need exists for cross-linkers which generate a high, thermally stable viscosity in a high pH environment.
Oil field service companies are currently using zirconium based cross-linkers to generate viscosity in polysaccharide-based fluids useful in hydraulic fracturing, completion and enhanced oil recovery applications. Commercially available, zirconium cross-linkers containing triethanolamine as a chelating ligand cross-link in the desired range and generate and maintain significant viscosity at 250° F. (121° C.), but at higher temperatures greater than or equal to 275° F., 135° C.) cross-link too fast. Replacement of triethanolamine with a hydroxyalkylated ethylenediamine chelating ligand such as in U.S. Pat. No. 4,798,902 gives a complex which cross-links too slowly at 250-275° F. (121-135° C.). The rate of cross-linking is critical to the successful generation of viscosity sufficient to conduct a fracturing operation.
Compositions are known in which aqueous zirconium compounds, triethanolamine and an α-hydroxycarboxylic acid are combined. Various processes have been used in combining these components to produce the compositions, such as those disclosed in U.S. Pat. Nos. 4,460,751; 5,182,408; and 5,798,320.
Some zirconium-based compositions of alkanolamine salts of α-hydroxycarboxylic acid may be used as cross-linkers in mid-high temperature fracturing fluid applications. While it is has been found that such triethanolamine zirconate complexes with α-hydroxycarboxylic acids have slower rates of cross-linking than similar complexes without α-hydroxycarboxylic acids, viscosity generation and retention are sacrificed, especially at temperatures greater than or equal to 275° F. (135° C.).
There is a need for zirconium-based cross-linking compositions which have a desirable 3-8 minute delay in rate of cross-linking without sacrificing the viscosity development capability of the zirconium composition at both lower mid temperature (up to 250° F., 121° C.) and at higher temperature (greater than 250° F., 121° C., especially greater than or equal to 275° F., 135° C.). That is, which will allow the cross-linking compositions to be used at a temperature greater than or equal to 250° F. (121° C.) and maintain adequate viscosity to ensure successful completion of the fracturing operation. The present invention meets these needs.